The present invention concerns a capacitively measuring sensor as a micromechanical semiconductor component, and a readout and evaluation circuit provided therefor.
For capacitively measuring sensors, such as for example pressure sensors and acceleration sensors, that have a high degree of sensitivity (that is, deliver a measurement signal that can be evaluated and is also very small in response to very small changes in a measurement quantity), evaluation circuits are required that convert the smallest changes in capacitance into an output signal that can be unambiguously allocated to the measurement quantity. Readout circuits are thus required that can be used for very small capacitances and changes thereof up to the region of a few 10 aF, and which are suited for monolithic integration together with capacitive sensors, which are realized in particular as a micromechanical component, preferably in All silicon. A capacitively measuring pressure sensor as a micromechanical component is specified for example in German reference DE 44 01 999 C2. A capacitively measuring acceleration sensor as a micromechanical component is for example specified in European reference EP 0 543 901 B1. The readout circuits should be capable of being coupled with evaluation circuits that convert an input signal into a bitstream or into a digital word that can be read in by a microprocessor. Due to the very small charge quantities that have to be detected, a technique is required that on the one hand does not falsify the signal and on the other hand is sufficiently sensitive to enable a reading out. For this purpose, conventional analog amplifiers are largely used, and the amplified signal is subsequently converted analog/digital. Isolated use is also made of .SIGMA.-.DELTA. modulators, as is specified for example in the article by C. Azeredo Leme and H. Baltes, "Interfaces for Microsensor Systems," in W. Sansen et al. (eds.), Analog Circuit Design, Kluwer Academic Publishers, Dordrecht 1994, pp. 163 to 181.